This invention relates to the area of oil and natural gas exploration and, more particularly, to a method for identifying regions of rock formations from which hydrocarbons may be produced.
Hydrocarbon saturation S.sub.o is generally determined from a measured water saturation S.sub.w as follows: EQU S.sub.o =1-S.sub.w (1)
Water saturation present in a subterranean formation is typically determined from interpretation of conventional electrical (i.e., resistivity) logs taken through a borehole drilled through the formation. Water saturation of the available pore space of the formation is determined from the resistivity log measurements using the Archie equation set forth in "The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics", Trans. AIME, Vol. 46, pp. 54-62, 1942, by G. E. Archie. This equation is expressed as follows: EQU S.sub.w.sup.n =R.sub.w /.phi..sup.m R.sub.t (2)
Where "S.sub.w " is the fractional water saturation (i.e. free and bound water of the formation expressed as a percent of the available pore space of the formation), "R.sub.w " is the formation water resistivity, ".phi." is the formation porosity, "R.sub.t " is the formation electrical resistivity, "n" is the saturation exponent and "m" is the porosity or cementation exponent. The Archie equation may be expressed in other ways and there are numerous methods in the art for determining, measuring or otherwise obtaining the various components needed to predict fractional water saturation S.sub.w from the formation resistivity, R.sub.t, using the equation in any of its forms.
Archie defined two quantities that provided the basis for his water saturation equation (1). The first quantity is the formation factor F which defines the effect of the rock matrix on the resistivity of water as follows: EQU F=R.sub.o /R.sub.w (3)
where
R.sub.o =resistivity of water saturated rock and PA1 R.sub.w =water resistivity.
Archie reasoned that for a given value of R.sub.w, the formation factor F would decrease with increasing porosity, .phi., to some exponent m: EQU F=1/.phi..sup.m (4)
This porosity exponent m has also become known as the Archie cementation exponent. Thus Archie provided a useful characterization of a rock fully saturated with a conducting brine in terms of the water resistivity R.sub.w, porosity .phi. and a rock parameter m. It is important to note that Archie assumed all conductance to be in the brine. PA3 where S.sub.w =volume of water in pores/total pore volume. This exponent n has become known as the Archie saturation exponent. It is again important to note that Archie assumed all conductance to be in the brine and further that all pores within the rock have the same water saturation S.sub.w.
The second quantity is the resistivity index I defined as the ratio of the resistivity of a rock partially saturated with water and hydrocarbon, R.sub.t, to the same rock saturated fully with water, R.sub.o, as follows: EQU I=R.sub.t /R.sub.o (5)
Archie reasoned that as the water saturation decreased (i.e. hydrocarbon saturation increased) the resistivity R.sub.t and hence I would increase to some exponent n: EQU I=1/S.sub.w.sup.n (6)
It is these two equations (4) and (6) for the formation factor F and resistivity index I respectively that Archie combined to provide the water saturation expression S.sub.w of equation (2). Certain logs have provided formation resistivity R.sub.t and porosity .phi.. Water samples provide the best values for R.sub.w. Standard practice is to measure rock sample resistivities R.sub.o and R.sub.t for a number of water saturations and to plot the logarithm of I versus the logarithm of S.sub.w. Archie's equations assume such a logarithmic plot is a straight line with slope of -n.
To determine the saturation exponent n, electrical resistivity measurements are carried out on core samples of porous rock at differing water saturation conditions. It is highly desirable for these water saturation conditions to be representative of those encountered in the formation. Different methods for desaturating core samples of porous rocks for electrical resistivity measurements are reviewed in "SCA Guidelines for Sample Preparation and Porosity Measurement of Electrical Resistivity Samples" by Maerefat et al., The Log Analyst, v.31, n.2, pgs. 68-75, March-April, 1990. One of the desaturating methods described is the one-phase displacement method in which water saturation in a core sample of a porous rock is altered by the injection of a second fluid phase. For example, a core sample may be saturated with conductive brine and oil injected as the displacing phase. This displacement is terminated at different stages, which equate to differing water saturation conditions, and electrical resistivity is measured. The fluids in the core sample are allowed to redistribute (i.e. equilibrate) after injection flow is discontinued before performing the electrical resistivity measurements.
One problem with such a one-phase desaturation method is that the electrical resistivity measurements can suffer from the fluids not redistributing to form a uniform water saturation at each of the differing desired water saturation conditions. also, the electrical resistivity measurements can suffer from capillary end effects due to capillary retention of water at the outflow end of the core sample. The end effect causes a sharp saturation gradient in the core sample, which may in some cases partially dissipate upon cessation of fluid flow. It is therefore an object of the present invention to provide for a one-phase method of desaturating a core sample of a porous rock for electrical resistivity measurements that overcomes such shortcomings of the above described one-phase desaturation method.